The present invention relates to feedthroughs for optical waveguides, and more particularly, to hermetically sealed feedthroughs suitable for use in high pressure, high temperature, and/or other harsh environments.
In many industries and applications, there is a need to have small diameter wires or optical waveguides penetrate a wall, bulkhead, or other feedthrough member wherein a relatively high fluid or gas differential pressure exists across the feedthrough member. In addition, one or both sides of the feedthrough member may be subjected to relatively high temperatures and other harsh environmental conditions, such as corrosive or volatile gas, fluids and other materials. In the case of electrical wires, these devices, called feedthroughs or penetrators, typically are constructed by using electrically conductive metal xe2x80x98pinsxe2x80x99 having a low thermal coefficient of expansion. The pins are concentrically located within a hole in a housing, and the resulting annular space is filled with a suitable sealing glass. Critical to the success of such seals is the selection and approximate matching of the thermal expansion rates of the various materials, i.e., the metal housing, sealing glass, and electrical pin. As the temperature range over which the feedthrough is exposed increases, the matching of thermal expansion rates becomes increasingly important in order to avoid failure of the feedthrough by excessive thermal stress at the interface layers between the various materials. This technology is relatively mature for electrical feedthroughs, and commercial devices are readily available that meet service temperatures in excess of 200xc2x0 C.
More recently, with the introduction of optical sensors, particularly sensors for use in oil and gas exploration and production, a need has emerged for a bulkhead feedthrough that can seal an optical fiber at high pressures of 20,000 psi and above, and high temperatures of 150xc2x0 C. to 250xc2x0 C., with desired service lives of 5 to 10 years. The sensing assembly of FIG. 3 is of the type disclosed in co-pending U.S. patent application Ser. No. 09/440,555 filed Nov. 15, 1999, entitled xe2x80x9cPressure Sensor Packaging For Harsh Environmentsxe2x80x9d, which is assigned to the Assignee of the present invention and is hereby expressly incorporated by reference as part of the present disclosure.
There are several problems associated with constructing such an optical fiber feedthrough. One of these problems is the susceptibility of the glass fiber to damage and breakage. This is due to the small size of the fiber, the brittle nature of the glass material, the susceptibility of the glass to stress corrosion cracking due to moisture exposure, and the typical presence of a significant stress concentration at the point at which the fiber enters and exits the feedthrough. Attempts to use a hard sealing glass, such as used with electrical feedthroughs, have had problems of this nature due to the high stress concentration at the fiber-to-sealing glass interface.
Another problem with sealing an optical fiber, as opposed to sealing an electrically-conductive metal xe2x80x98pinxe2x80x99 in an electrical feedthrough, is that the fused silica material of which the optical fiber is made, has an extremely low thermal expansion rate. Compared to most engineering materials, including metals, sealing glasses, and even the metal xe2x80x98pinsxe2x80x99 typically used in electrical feedthroughs, the coefficient of thermal expansion of the optical fiber is essentially zero. This greatly increases the thermal stress problem at the glass-to-sealing material interface, particularly as the application temperatures rise.
One technique used to produce optical fiber feedthroughs is the use of a sealed window with a lensing system. In this technique, the optical fiber must be terminated on each side of a pressure-sealed window, thus allowing the light to pass from the fiber into a lens, through the window, into another lens, and finally into the second fiber. The disadvantages associated with this system include the non-continuous fiber path, the need to provide two fiber terminations thus increasing manufacturing complexity, and the light attenuation associated with these features.
Another approach to producing optical fiber feedthroughs involves passing the fiber through a bulkhead without termination, while providing a seal around the fiber to prevent leakage across the bulkhead. One such seal has been implemented by means of a sapphire compression fitting to take advantage of the pressure differential typically present across a bulkhead in a harsh environment. One disadvantage associated with this type of seal, however, is that it has been found to suffer from creep of material across the bulkhead in the direction of the decreasing pressure gradient, which can, in turn, compromise both the optical fiber and seal.
It is often desirable to mount fiber optic based sensors in harsh environments that are environmentally separated from other environments by physical bulkheads. An exemplary such fiber optic based sensor is disclosed in co-pending U.S. patent application Ser. No. 09/205,944 entitled xe2x80x9cTube-Encased Fiber Grating Pressure Sensorxe2x80x9d to T. J. Bailey et al., which is assigned to the Assignee of the present invention and is hereby expressly incorporated by reference as part of the present disclosure. This exemplary optical sensor is encased within a tube and certain embodiments are disclosed wherein the sensor is suspended within a fluid. Some such fiber optic sensors have sensors and tubes that are comprised of glass, which tends to be relatively fragile, brittle and sensitive to cracking. Thus, the use of such a sensor in a harsh environment, such as where the sensor would be subjected to substantial levels of pressure, temperature, shock and/or vibration, presents a significant threat of damage to the sensor. In certain environments, such sensors are subjected to continuous temperatures in the range of 150xc2x0 C. to 250xc2x0 C., shock levels in excess of 100 Gs, and vibration levels of 5G RMS at typical frequencies between about 10 Hz and 200 Hz and pressures of about 15 kpsi or higher.
However, as discussed above, the harsh environments where the sensors are located generally must be isolated by sealed physical barriers from other proximate environments through which the optical fiber communication link of the sensor must pass. It is important to seal the bulkhead around the optical fiber to prevent adjacent environments from contamination, as well as to protect the optical fiber as it passes through adjacent environments. If the optical fiber is compromised by contamination from an adjacent harsh environment, the optical fiber and all sensors to which it is connected are likely to become useless.
Accordingly, it is an object of the present invention to provide an optical waveguide feedthrough assembly, and a method of making such an assembly, which overcomes one or more of the above-described drawbacks and disadvantages of the prior art, and is capable of relatively long-lasting operation at relatively high pressures and/or temperatures.
The present invention is directed to an optical waveguide feedthrough assembly for passing at least one optical waveguide, such as an optical fiber, through a sensor wall, bulkhead, or other feedthrough member. The feedthrough assembly of the present invention comprises a tubular member or like support defining an axially elongated, annular surface, wherein the annular surface forms an axially elongated optical feedthrough cavity. The optical fiber or like waveguide is received through the axially-elongated optical feedthrough cavity, and is spaced radially inwardly relative to the annular surface to thereby define an axially-elongated annular cavity between the fiber and annular surface. A sealant, such as an epoxy adhesive, is received within and substantially fills the annular cavity. The sealant exhibits adhesive properties at the interface of the sealant and optical fiber, and at the interface of the sealant and the annular surface, to adhesively secure and hermetically seal the optical fiber within the feedthrough cavity and substantially prevent axial movement of the sealant and optical fiber relative to the annular surface.
The optical feedthrough cavity is defined by an outer dimension having one or more variations along the axial direction thereof, and the dimensional variations cooperate with the sealant to further prevent axial movement of the sealant relative to the annular surface. In accordance with an embodiment of the present invention, the annular surface of the tubular member defines one or more annular constrictions or like radially projecting interruptions forming the variations in the outer dimension of the annular cavity for further preventing movement of the epoxy or like sealant plug in the axial direction.
The present invention is also directed to a method of making an optical feedthrough assembly, including the following steps: (a) forming the annular cavity of the tubular member with a predetermined width between the optical fiber and the annular surface to allow the epoxy or other sealant in its liquid phase to substantially fill the annular cavity by capillary action; (b) selecting a polymeric or other type of sealant capable of exhibiting a viscosity which allows the sealant to substantially fill the annular cavity by capillary action, and also capable of exhibiting a viscosity which substantially prevents leakage of the sealant out of the ends of the annular cavity upon filling the cavity; (c) introducing the polymeric or other sealant in its liquid phase into the annular cavity and allowing the sealant to substantially fill the annular cavity by capillary action; and (d) wherein upon filling the annular cavity, the polymeric or like sealant transitions to its solid phase and adhesively secures the fiber within the optical feedthrough cavity, and substantially prevents movement of the solid epoxy or sealant plug out of the cavity.
One advantage of the method and assembly of the present invention is that they are capable of providing an optical feedthrough assembly with minimal leakage and high longevity in relatively high pressure, high temperature and other harsh environments.
Another advantage of the method and assembly of the present invention is that they enable the use of polymeric or like sealants having low elastic moduli to thereby significantly improve the resistance of the glass fiber to damage and breakage. Epoxies or like sealants further provide a natural strain relief at the interface between the glass fiber and the feedthrough assembly at the points where the fiber enters and exits the feedthrough. Accordingly, the feedthrough assemblies of the present invention may exhibit significantly lower stress concentrations and improved survivability in comparison to the prior art feedthroughs described above.
Another advantage of the method and assembly of the present invention is that they enable the use of a polymeric or like sealant having a relatively low elastic modulus to minimize any thermal stress at the interface of the optical fiber or like waveguide and feedthrough assembly. As a result, the present invention substantially avoids the problems encountered in the above-described prior art feedthroughs wherein significant thermal stresses are created at the interfaces of the optical fibers and feedthroughs due to the extremely low rate of thermal expansion of the optical fiber material in contrast to the adjoining material of the prior art feedthroughs.
A further advantage of the method and assembly of the present invention is that the feedthrough assembly may form a continuous (or uninterrupted) fiber or like waveguide path from one end of the assembly to the other. As a result, there is essentially zero light attenuation when using, for example, single mode fiber with a high numerical aperture (NA). Such high NA single mode fiber, sometimes called xe2x80x98bend-insensitivexe2x80x99 fiber, is typically used in Bragg grating-based optical fiber sensors employed in oil and gas exploration and production, where the low light attenuation properties of the fiber are particularly useful in such systems having sensors located at great distances from the light source which interrogates the sensor.
These and other objects and advantages of the present invention will become readily apparent in view of the following detailed description of preferred embodiments and accompanying drawings